1. Field of the Invention
The present invention relates to apparatus and methods for adaptively reducing dead time in switching circuits.
2. Description of the Prior Art
FIG. 1 (prior art) shows a power output stage, typical for a switching audio amplifier. PWM Control Block 101 is the control circuitry that changes the input (in this case audio) into pulse width modulated control signals. The input may be analog or digital. Drivers 102 and 103 turn on and off the gates of switches 104 and 105. In this case, the switches are FETs, but other active devices may be used. The use of FETs is typical for audio amplification. Filter 106, normally an LC lowpass filter, removes the switching frequencies from the signal. Load 107 receives the power; in the audio case this is a speaker system. Snuber network 108 is optional, and is used to control the switching waveshape. Catch diodes 110 and 111 control the range of voltages during transitions, as filter 106 is usually inductive. These catch diodes are often integrated into transistors 104 and 105.
It is important that output devices 104 and 105 not be on, or in conduction, at the same time, as high current will flow between the supplies and through the devices, causing a lack of efficiency, or even destruction of the devices. A dead time, meaning a time when neither device is on, is used to guarantee this does not occur. FIG. 2 (prior art) illustrates this principal. Each switch 104, 105 is turned off for a period 201 before the other is turned on. Period 201 is called the dead time.
It is important that dead time 201 be as small as possible, without overlap of on signals occurring. During the dead time, the voltage to the filter is not well controlled. This causes distortion in the output. In current designs, using very high drive current in drivers 102 and 103, and making the switching times very short minimizes dead time. This approach has the undesirable effect of increasing the demands on the power supply and wiring, and also emits more RF radiation.
A need remains in the art for apparatus and methods for minimizing the dead time in switching circuits, without causing overlap of the on conduction in the switches.